The present invention is directed to a process for reducing the sulfur content in gasoline to a very low level. Gasoline is generally prepared from a number of hydrocarbonaceous blend streams and typical examples include butanes, light straight run naphtha, isomerate, FCC cracked gasoline, hydrocracked naphtha, coker gasoline, alkylate, reformate, ethers and alcohols. Of these, gasoline blend stocks produced in a fluid catalytic cracking unit (FCC), the reformer and the alkylation unit account for a major portion of the gasoline pool. FCC gasoline, and if present, coker naphtha and pyrolysis gasoline, generally contribute a substantial portion of the gasoline pool sulfur.
Sulfur present in the gasoline pool may be in one of several molecular forms, including thiophenes, mercaptans, sulfides and disulfides. Typical thiophenes include thiophene and its alkylated derivatives and benzothiophene and its alkylated derivatives. Typical mercaptans occurring in the sulfur-containing gasoline streams include thiophenol and the alkyl thiols from ethane thiol to nonanethiol, with potentially smaller amounts of the higher alkyl thiols.
A number of methods have been proposed for removing sulfur from gasoline. In general, hydrotreating is the method of choice, because of the cost and ease of processing using the catalytic hydrotreating method. However, sulfur removal by hydrotreating is generally accompanied by substantial octane loss when the olefins in gasoline are converted to low octane components while the sulfur compounds are simultaneously being removed. A number of proposals have been made to offset the octane loss associated with gasoline hydrotreating.
As evidenced from the hereinabove, it is clear that many approaches have been utilized in order to reduce the sulfur level in gasoline. However, new government regulations which require ultra low sulfur levels in gasoline have been promulgated and will be coming into effect soon. Even though very low sulfur levels are desired, there continues to be a need for gasoline which has a high octane rating. With these often-conflicting objectives, it is apparent that there is a need for new methods for reducing sulfur levels in a gasoline pool while maintaining the pool octane rating.
According to U.S. Pat. No. 3,957,625 B1, the sulfur impurities tend to concentrate in the heavy fraction of the gasoline and a method for removing the sulfur includes hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction.
U.S. Pat. No. 6,228,254 B1 (Jossens et al) discloses a two-step sulfur removal process comprising a mild hydrotreating step followed by an extraction step to reduce the sulfur content in gasoline to a very low level without significantly reducing the octane of the gasoline.
U.S. Pat. No. 5,582,714 B1 (Forte) discloses a process for the removal of sulfur from FCC gasoline by employing a solvent. Preferred solvents are glycols and glycol ethers.
U.S. Pat. No. 2,634,230 B1 (Arnold et al) discloses a process for the desulfurization of high sulfur olefinic naphtha wherein 2,4-dimethyl sulfolane is employed to extract sulfur from a highly olefinic naphtha, such that the solvent does not effect separation between olefins and paraffins, to produce a sulfur lean raffinate phase and a sulfur rich extract.
A paper titled, xe2x80x9cRemoval of Sulfur From Light FCC Gasoline Streamxe2x80x9d Presented at the NPRA 2000 Annual Meeting Mar. 26-28, 2000 in San Antonio, Tex. discloses that sulfur compounds in the initial boiling range of light FCC gasoline are primarily mercaptans which are caustic extractable.
A paper titled, xe2x80x9cNovel Process For FCC Gasoline Desulfurization and Benzene Reduction to Meet Clean Fuels Requirementsxe2x80x9d Presented at the NPRA 2000 Annual Meeting, Mar. 26-28, 2000 in San Antonio, Tex. discloses that sulfur and aromatic species in FCC naphtha may be segregated by using solvent extraction.
None of the cited references disclose a three-way splitter with the extraction of thiophene from the mid boiling fraction.
The present invention is a process for desulfurizing a gasoline stream while continuing to maintain the octane rating of the blend stock. In accordance with the process of the present invention, a gasoline stream containing sulfur compounds and olefins is introduced into a fractionation zone to produce a low boiling fraction containing mercaptan sulfur compounds and olefins, a mid boiling fraction containing thiophene and olefins, and a high boiling fraction containing sulfur compounds. The low boiling fraction containing mercaptan sulfur compounds is, in one embodiment, contacted with an aqueous alkaline solution to selectively remove at least a portion of the mercaptan sulfur compounds. The mid boiling fraction containing thiophene and olefins is contacted with a lean solvent to produce a raffinate stream containing olefins and having a reduced sulfur content relative to the mid boiling fraction and a rich solvent stream enriched in the thiophene. The rich solvent stream enriched in thiophene is separated to produce a hydrocarbonaceous stream rich in thiophene. In another embodiment, the thiophene is removed from the mid boiling fraction containing thiophene and olefins by extractive distillation to produce a raffinate stream containing olefins having a reduced sulfur content relative to the mid boiling fraction and a hydrocarbonaceous stream rich in thiophene. The resulting hydrocarbonaceous stream rich in thiophene and the higher boiling fraction containing sulfur compounds is reacted in a hydrodesulfurization reaction zone to produce a hydrocarbonaceous stream having a reduced sulfur concentration.
In accordance with one embodiment, the present invention relates to a process for desulfurizing gasoline containing olefins comprising the steps of: (a) introducing a gasoline stream comprising sulfur compounds and olefins into a fractionation zone to produce a low boiling fraction comprising mercaptan sulfur compounds and olefins, a mid boiling fraction comprising thiophene and a high boiling fraction comprising sulfur compounds; (b) contacting the low boiling fraction comprising mercaptan sulfur compounds with an aqueous alkaline solution to selectively remove at least a portion of the mercaptan sulfur compounds; (c) removing at least a portion of the thiophene in the mid boiling fraction to produce a raffinate stream having a reduced sulfur content relative to the mid boiling fraction and an extract stream enriched in thiophene; (d) separating the extract stream enriched in thiophene to produce a hydrocarbonaceous stream rich in thiophene; (e) reacting the hydrocarbonaceous stream rich in thiophene recovered in step (d) and the high boiling fraction comprising sulfur compounds recovered in step (a) in a hydrodesulfurization reaction zone to produce a hydrocarbonaceous stream having a reduced sulfur concentration; and (f) recovering a desulfurized gasoline comprising olefins.
In accordance with another embodiment, the present invention is a process for desulfurizing gasoline containing olefins comprising the steps of: (a) introducing a gasoline stream comprising sulfur compounds and olefins into a fractionation zone to produce a low boiling fraction comprising mercaptan sulfur compounds and olefins, a mid boiling fraction comprising thiophene and a high boiling fraction comprising sulfur compounds; (b) contacting the low boiling fraction comprising mercaptan sulfur compounds with an aqueous alkaline solution to selectively remove at least a portion of the mercaptan sulfur compounds; (c) contacting the mid boiling fraction comprising thiophene with a lean solvent to produce a raffinate stream having a reduced sulfur content relative to the mid boiling fraction and a rich-solvent stream enriched in the thiophene; (d) separating the rich-solvent stream enriched in thiophene to produce a hydrocarbonaceous stream rich in thiophene; (e) reacting the hydrocarbonaceous stream rich in thiophene recovered in step (d) and the high boiling fraction comprising sulfur compounds recovered in step (a) in a hydrodesulfurization reaction zone to produce a hydrocarbonaceous stream having a reduced sulfur concentration; and (f) recovering a desulfurized gasoline comprising olefins.
And in another embodiment the present invention is a process or desulfurizing gasoline containing olefins comprising the steps of: (a) introducing a gasoline stream comprising sulfur compounds and olefins into a fractionation zone to produce a low boiling fraction comprising mercaptan sulfur compounds and olefins, a mid boiling fraction comprising thiophene and a high boiling fraction comprising sulfur compounds; (b) contacting the low boiling fraction comprising mercaptan sulfur compounds with an aqueous alkaline solution to selectively remove at least a portion of the mercaptan sulfur compounds; (c) introducing the mid boiling fraction comprising thiophene into an extractive distillation zone to produce a raffinate stream having a reduced sulfur content relative to the mid boiling fraction and a hydrocarbonaceous stream rich in thiophene; (d) reacting the hydrocarbonaceous stream rich in thiophene recovered in step (c) and the high boiling fraction comprising sulfur compounds recovered in step (a) in a hydrodesulfurization zone to produce a hydrocarbonaceous stream having a reduced sulfur concentration; and (e) recovering a desulfurized gasoline comprising olefins.
Other embodiments of the present invention encompass further details such as types and descriptions of feedstocks, catalysts, solvents and preferred operating conditions including temperatures and pressures, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.